1. Field of Invention
The present invention relates to a three-dimensional (3D) stereoscopic imaging technology, and more particularly to a 3D stereoscopic imaging method having a real physical depth of field, a system thereof and an imaging device thereof.
2. Description of Related Arts
Different from the two-dimensional (2D) plan imaging technology, the 3D stereoscopic imaging technology compresses 3D image information into a 2D plan, which necessarily causes an image distortion and fails to accurately show a real spatial location of each pixel in the image. A 2D plan displays a 3D image through brightness of colors, sizes of objects and so on. Based on psychological cues and information including brightness of colors and sizes of objects, people subjectively judge distances between each pixel on the 2D plan and human eyes, rather than a real physical depth of field.
The 3D display, different from the 2D display, brings viewers visual depth perception through various ways in such a manner that the viewers naturally or unnaturally obtain information about a third dimension of a picture. This perceiving method defines a difference between a real 3D and a fake 3D for the human eyes. Thus, in the 3D stereoscopic imaging technology, it is very important to restore the real physical depth of field in a 3D stereoscopic space and also it is the most key factor for the human eyes to perceive a 3D stereoscopic image.
Physiology studies indicate that human eyes obtain the 3D stereoscopic perception of the objective world mainly through following four effects.
(1) Accommodation Effect
The accommodation effect means that the human eyes adjust focuses of the crystal bodies in the eyeballs through ciliary muscle contraction. Obviously, the accommodation effect still exists even in a monocular observation, which belongs to a psychological cue of monocular depth perception. However the psychological cue of the monocular depth perception works only under cooperation with a binocular psychological cue and when the observed object is relatively close to the human eyes.
(2) Convergence Effect
The convergence effect refers that when two eyes are observing a point of an object, visual axes of the two eyes form a convergence angle. Obviously, the ciliary muscle contracts to turn the eyeball inward slightly so that a psychological cue of depth perception is generated when two eyes are focused on the point. The binocular psychological cue is called convergence effect. Usually the accommodation effect and the convergence effect are related with each other. The convergence effect becomes obvious only when the observed object is close to the human eyes.
(3) Binocular Disparity
Two human eyes are spatially separated by an interpupillary distance around 6.5 cm. When the two eyes are observing a 3D stereoscopic object, the two eyes stand from slightly different angles and thus views from the two eyes are slightly different, which is called the binocular disparity. For objects provided at a medium sight distance, the binocular disparity information is acknowledged by people as the most important psychological cue in the depth perception. When two eyes are observing a point of an object, the point in space projects to the centers of the retinas of the two eyes. Thus, the two centers of the pair of eyes generate “corresponding positions” on the retinas and further a convergence angle is decided by the “corresponding positions”. Light projected from many other points except the view point does not always fall on the two retinal “corresponding positions”. This is defined as the binocular disparity. This basic principle is applied in various modern technologies which generates 3D stereoscopic images through 2D plan images.
(4) Monocular Motion Parallax
When an object is observed only with one eye, if the eye keeps stationary, the accommodation effect is the only psychological cue to obtain depth perception; if the observer moves, the object is observed from all directions based on the binocular parallax to provide cues for the depth perception, which is called the monocular motion parallax. Obviously, the monocular motion parallax does not work for static objects.
The human eyes watches a reconstructed image of holography just like watching a real 3D object, wherein all the above four effects exist. Thus the human eyes are watching naturally.
Nowadays, the “3D stereoscopic display technology” includes the glasses-wearing stereo technology and the glasses-free stereo technology. When people are watching a 3D movie (the common glasses-wearing 3D movie), only the binocular parallax exists. Even though the binocular parallax is a very key psychological cue to obtain depth perception according to physiology, an absence of other psychological cues results in a nervous state of human eyes rather than a very natural state. The nervous state remains unobvious in a short period of watching static stereoscopic image, but when watching a stereoscopic television set, the unnaturally watching state of the human eyes lasts in such a long time that people may feel very uncomfortable and very fatigued. Thus a pair of red-blue glasses or a pair of light-polarizing glasses produces a type of “eyes-deceived fake stereoscopic effects” and fails to well restore real spatial perception.
The glasses-free stereoscopic display technology includes a holographic 3D display technology, a volumetric 3D display technology and so on.
Under a principle that a specular reflection produces a mirror image, the holographic 3D technology produces very lifelike stereoscopic effects. However, a dynamic display requires a very high spatial light modulator and a super-high-speed data processing system, which limit developments and applications into daily life of the holographic 3D technology. In the patent US2008/0144175 A1, VIZOO Invest ApS discloses a pyramid-like 360 degrees display device. Based on the principle that reflection produces a mirror image, the device uses four reflective mirrors to image a plan image above or below the reflective mirror at a center of the pyramid-like structure to realize the 360 degrees displaying angle. A simple device like this still projects plan images, but provides viewers with cues of “a virtual depth of field” or “a psychological depth of field”. However, the device can not provide viewers with real depth information.
Different from other stereoscopic display technologies, the volumetric display technology is really able to realize dynamic effects and gives viewers an observation of 3D perspective images “suspending” in the air, like in science fiction films. Conventionally, the volumetric 3D display technologies include a swept-volume display technology and a solid-volume display technology.
The swept-volume display technology uses structures of cylinder axis rotation plus spatial projection and includes an upright projection screen driven by a motor to rotate at a high speed and very thin semitransparent plastic. In order to display a 3D object, firstly several slices of the object are generated by software, wherein each longitudinal slice vertical to X-Y plane is extracted during a rotation of every tiny angle along Z-axis, and then, when the projection screen rotates rapidly and the plurality of slices are rapidly projected in turn, a natural 3D object is able to be perceived from all directions. The swept-volume display technology has obvious disadvantages of “luminosity” and “rotation”. The display structures of all-direction openness and projection has a relatively low lumen value and tends to be affected by background lightning; the rapid rotation gives a settling platform a high requirement of stability and the settling table is forbidden to shake casually, otherwise voxels may be displayed vaguely even imaging may totally fails. Thus the swept-volume display technology is unfit for places such as spacecrafts, aircrafts and sailing vessels. Meanwhile, the imaging process is complicated and the costs are too expensive to be applied in a large scale.
As an early solid-volume display technology, solid FELIX mainly uses a whole piece of cubical crystal as a display medium. The crystal is doped with rare earth elements. When two cohered infra-red laser beams intersect at a spatial point inside the crystal, the spatial point is excited to radiate light. The solid FELIX still remains experimental in the lab. Depth Cube in considered as the highest achievement in the conventional solid-volume display technology. The Depth Cube system adopts a special method of stacking liquid crystal screens to realize 3D display and thus looks like a television set in 1980s. The display medium of the Depth Cube includes a stack of 20 liquid crystal screens, wherein each screen has a resolution of 1024×748; each screen is spaced around 5 mm. Liquid crystal pixels of these specially made screen bodies have a special electronically-controlled optical property. Under a charge of voltage, the liquid crystal body of the pixel becomes paralleling with a light beam transmission manner just like a slat of a window shades and thus the beam shot thereon perforates transparently; without any charge of voltage, the liquid crystal pixel becomes opaque and thus the beam shot thereon is diffusely reflected to form a voxel existing in the stacking body of liquid crystal screens. At any instant in time, 19 liquid crystal screens are transparent and only one is opaque and in a white diffuse reflection state; Depth Cube provides depth feelings through rapidly switching and displaying the slices of the 3D object on the 20 screens. The Depth Cube has a relatively limited viewing angle and thus is usually applied in a front face of a monitor.
The above volumetric 3D display technologies mostly use the projectors, but each pixel of a 3D image are not self-luminous. Light projected by the projector is shot onto a projection screen to light all the pixels. A quality of an image shown by the volumetric 3D display technologies is closely related with projection effects of the projection screen.
Meanwhile, all the above volumetric 3D display technologies have an inevitable disadvantage of only generating a semitransparent 3D perspective image to display an object at 360 degrees, because lights from a front of an object are unable to stop a transmission of lights from a back of the object.
Although the above three display technologies can realize real stereoscopic effects, complicated structures thereof result in expensive prices. Thus the above three display technologies are only applied in relatively advanced industries, such as laboratories, medical devices and military, rather than widely promoted into daily life.